The invention relates to optical analyzing instruments, more particularly to reflective or transmissive optical analyzing instruments with the purpose of determining the composition of solid, liquid, slurry or paste samples by their near-infrared, infrared or visible absorbances with special respect to industrial on-line monitoring applications.
The background of the invention is the empirical correlation near-infrared reflectance analysis method first suggested and elaborated by Karl H. Norris of the USDA, Beltsville, Md., in the mid-sixties. It was observed, that if different wavelength near-infrared radiation is incident on the surface of the sample it is absorbed or reflected to certain extents, depending on the characteristics, and thus the concentrations of the constituents of the sample. The reflected radiation can be collected by suitable optics and the radiation can be measured by a suitable detector arrangement. The concentrations of the material to be measured can be calculated from the intensities measured at different wavelengths. For a more detailed history and background see the article by David L. Wetzel published in Analytical Chemistry Vol. 55, p. 1165A (1983).
For the above purposes, in the past various instruments comprising different monochromators have been reported. Most systems were aimed at laboratory analysis of samples and it is difficult to apply the instrumental principles and systems in the prior art for industrial on-line concentration monitoring. Typically in on-line monitoring the samples are moving , in many cases they are inhomogeneous, thus to achieve the monitoring goals a large number of measurement should be done in a very short time, and the results averaged to reduce errors. In addition to the averaging requirement it would be needed to correct for the rapid changing of the sample, and possibly use optical data that were taken from the same part of the sample, in the calibration equation. This can be achieved in principle by applying a "stopped-flow" sampling, but this procedure is less representative to the bulk of the sample and also much too slow for most control purposes.
In the prior art interference filter systems with a perpendicular direction of light beam are described in U.S. Pat. Nos. 3,828,173 and 4,236,076. In both systems discrete wavelength interference filters are mounted in a turret, thus the rate of the wavelength selection is limited by the mechanical means to rotate the turret.
A special mention among interference filter instruments is deserved for a system in the prior art, described in U.S. Pat. Nos. 4,286,327 and 4,404,642. As light sources infrared emitting diodes (IREDs) are used, whereas all other instruments in the prior art utilize wide wavelength band quartz tungsten-halogen light sources. The advantage of the special light source is that it dissipates only a fraction of what conventional light sources do, and can be activated very rapidly by a timer through a microcomputer. In return for these advantages the wavelength region is constrained to the region of the IREDs (about 850-1050 nm). In this region a very sensitive Si detector can be used but only a few chemical components show characteristic absorbances.
Tilting interference filter systems are described in U.S. Pat. Nos. 3,861,788 and 4,082,464. The interference filters are mounted on paddle wheels, and rotated to result in a wavelength shift as the angle of the filter and the incident beam varies in time. These systems produce continuous wavelength change, but only very small fractions of the whole rotation can be considered "useful" time, when the filter is exactly producing the required wavelengths. There is also considerable dead time, when the beam is mechanically blocked between filters.
Diffraction grating systems for optically analyzing samples have been described in the prior art. Vibrating holographic grating systems, capable of up to ten scans per second were presented in U.S. Pat. Nos. 4,264,205, 4,285,596 and recently in U.S. Pat. No. 4,540,282. The vibrating grating principle allows only sequential access to the individual wavelengths as the whole spectrum is swept through in time. One of the disadvantages in the high speed applications of grating monochromator is that the signal-to-noise ratio cannot be enhanced by chopping and narrow noise bandwidth phase sensitive (lock-in) amplification. This technique can only be applied in slow point-to-point scan grating systems, where the grating is stopped at every required wavelength, and enough light chopping periods are allowed to elapse for the signal intensity to be precisely measured. Thus the total spectrum measurement time is increased up to about a minute.
Another trade-off in mechanical (rotating sector) chopper systems is, that only half of the measurement time is spent by the detector collecting light at the required wavelenth, in the other half of the period a fast dark compensation occurs, carrying no "wavelength information". Lock-in amplification is described in U.S. Pat. No. 4,236,076, where the light is modulated by chopping the light, periodically referencing by a tilting mirror and changing the wavelength by changing interference filters in the beam. A disadvantage of this system for the monitoring of rapidly changing material is that the wavelength change is the slowest of the mentioned three modulation, the rate being several seconds between consecutive wavelength choices.
The further increase of the speed, improvement of the efficiency of the monochromator and the analyzer system runs into serious problems with the mechanical monochromatic systems in the prior art. From among the non-mechanically tunable optical devices acousto-optical tunable filters (AOTF) have been described by S. E. Harris and R. W. Wallace in the Journal of the Optical Society of America, Vol. 59, pp. 744-747 (1969). This early model was tunable from 400 nm to 700 nm by applying 428-990 MHz acoustic frequency via an acoustic transducer layer attached to a LiNbO crystal. Since then various acousto-optic devices have been described by I. C. Chang in Applied Physics Letters Vol. 25, pp. 370-372 (1974) and also in U.S. Pat. Nos. 3,679,288; 3,944,334; 3,944,335; 3,953,107; 4,052,121 and 4,342,502. In the above mentioned disclosures the geometry, the material of the crystal used and the optical arrangement varies, but in common all acousto-optic tunable filters utilize the principle that the direction of propagation and the direction of polarization of an appropriate incident ray is changed by the application of a high frequency optical modulation of the crystal. The different frequencies give rise to different densities of index of refraction fronts due to local stresses caused by the acoustic waves. This tuning can be activated over a relatively wide frequency(wavelength) range, thus rendering the device ideal for optical tuning purposes. If the wideband input light is polarized, another polarizer (the so called analyzer) can select the tuned monochromatic ray from the traversing untuned polychromatic light. FIG. 1 shows a possible optical arrangement of an AOTF commercially available. Since the wavelength accessibility is influenced by the size and geometry of the acousto-optic crystal and the velocity of the sound travelling in the crystal, a 100-100,000 fold advantage in the speed of the wavelength change rate may be obtained compared to other analyzers in the prior art.